The present invention relates generally to an optical disk driving apparatus. More particularly, the invention relates to a control system for an optical disk driving apparatus which is adapted to rotate an optical disk having a plurality of tracks, and which is adapted, during the rotation of the disk, to perform optical recording and/or reproduction of information for a track which has been accessed by a light beam impinging. It is to be noted that, in the present invention, the term "optical disk" means various kinds of optical disks such as, for example, a read-only optical disk, or a rewritable or erasable magneto-optical disk.
There are two known kinds of control systems, for defining the relationship between the rotation of an optical disk and the recording format of information on the optical disk. One is a constant angular velocity (CAV) type and the other is a constant linear velocity (CLV) type.
In an optical disk driving apparatus using a CAV system, when optical recording and/or reproduction of information for an optical disk is performed, the optical disk is rotated at a constant angular velocity. That is, the number of revolutions per unit time (i.e., rotation speed) of the optical disk is maintained constant, without relation to the radius of the tracks formed on the optical disk. Therefore, when the rate of recording and/or reproduction of information is also maintained constant, the information recording density on a given track is different from that on the other tracks, due to different linear velocities on the respective tracks, resulting in a decrease of the recording capacity of the optical disk.
On the other hand, in an optical disk driving apparatus using a CLV system, when optical record and/or reproduction of information for an optical disk is performed through an optical pickup, the rotation speed of the optical disk is varied in relation to the radii of tracks formed on the optical disk so that a constant linear velocity of the optical disk with respect to the optical pickup can be obtained on all the tracks.
It is known that when the speed of recording/reproduction of information for the optical disk is given as f (bit/sec); the radius of a track to be accessed to record and/or reproduce information is given as r (mm); and the speed of rotation of the optical disk is given as n (rps), then the information recording density b (bit/mm) on the track of the optical disk is represented by the following equation. EQU b=f/(2.pi..multidot.n.multidot.r) (1)
In an optical disk driving system using a CLV system, in order to obtain a constant information recording density b on all the tracks of the optical disk, the rotation speed n of the optical disk is changed in inverse proportion to the radius r of the tracks, while the speed f of record/reproduction of information is maintained constant.
In an optical disk driving apparatus for playback-only applications, such as a compact disk (CD) player or the like, the rotation speed of an optical disk is automatically changed by controlling the driving force of an electric drive motor for rotating the optical disk, in order to obtain a reproduction signal at a constant speed.
On the other hand, in an optical disk driving apparatus adapted to perform both optical recording and/or reproduction of information for an optical disk, it is impossible to obtain a reproduction signal with a constant speed at or before the time of information is perfomred. Accordingly, generally, recording the latter system changes the rotation speed of the optical disk in steps with a relatively high resolution by using a signal output from an encoder secured to the drive motor, and the change in the rotation speed of the optical disk is regarded as a sufficiently continuous change.
In the CLV system, the recording of information is performed with a constant recording density b of information, and accordingly, it is possible to increase the recording capacity of the optical disk in comparison with that obtained by the CAV system. However, in the CLV system, it is necessary to change the rotation speed of the optical disk in accordance with the radius r of a track to be accessed for the recording and/or reproduction of information. This means that in order to effect quick access between the two tracks having significantly different radii with respect each other, it is necessary to produce a quick and significant change of the rotation speed of the optical disk. Accordingly, for the above-mentioned quick and significant change, it is necessary to use a drive motor having a large torque, causing the drive motor as well as the optical disk driving apparatus to be increased in size. For the above-mentioned accessing operation it is also necessary to supply a large amount of electric current to the drive motor, resulting in an increased electric power consumption.
On the other hand, as a method for maintaining a constant information recording density b on all the tracks of the optical disk while keeping a constant rotation speed n of the optical disk, it is possible to consider a construction in that the speed f of recording/reproduction of information is changed in proportion to the radius r of a track to be accessed, on the basis of a signal output from an encoder secured to the drive motor. However, in fact it is difficult to change the speed f of recording/reproduction of information with a necessary high resolution such that the change in the speed f of record/reproduction of information can be regarded as a sufficiently continuous change in comparison with the rotation speed n of the optical disk. Accordingly, it is difficult to realize the characteristics as described above, becuase the frequency of the information recording and/or reproduction signal is extremely high in comparison with that of the signal output from the encoder.